1. Field of the Invention
The present invention relates to a glass substrate polishing method, a package manufacturing method, a piezoelectric vibrator, an oscillator, an electronic device and a radio timepiece.
2. Description of the Related Art
In recent years, mobile telephones and portable information terminal devices employ a piezoelectric vibrator that uses crystal or the like as a time source, a timing source of control signals or the like, and a reference signal source etc. Various examples of this type of piezoelectric vibrator are known. One known example is a surface mount device (SMD) type piezoelectric vibrator. This type of piezoelectric vibrator includes, for example, a base substrate and a lid substrate that are bonded to each other, a cavity that is formed between the two substrates, and a piezoelectric vibrating reed (an electronic component) that is housed in the cavity in an airtight sealed state.
This type of piezoelectric vibrator has a two-layer structure in which the base substrate and the lid substrate are directly bonded to each other, and the piezoelectric vibrating reed is housed in the cavity formed between the two substrates.
When this type of piezoelectric vibrator having the two-layer structure is manufactured, after the piezoelectric vibrating reed is mounted on the base substrate while a recessed portion for the cavity is formed on the lid substrate, both wafers are anodically bonded via a bonding layer. As a result of this, a plurality of piezoelectric vibrators (a package) is manufactured, in which the piezoelectric vibrating reed is airtightly enclosed in the cavity.
Patent Document 1: JP-A-5-177539
In the above-mentioned manufacturing method of the piezoelectric vibrator, when the base substrate and the lid substrate are bonded, it becomes important to bond flat surfaces of the two substrates while securing bonding areas (areas of bonding surfaces) and surface accuracy of the bonding surfaces of the two substrates, in order to maintain airtightness in the cavity.
Hence, when the two substrates are bonded, a polishing process is performed in advance in which the bonding surfaces of the two substrates are polished to improve the surface accuracy. One example of the polishing process is, for example, a polishing method that is disclosed in JP-A-5-177539 in which a carrier that retains a substrate is sandwiched between a pair of upper and lower surface plates that are respectively fitted with a polishing cloth, and both surfaces of the substrate are polished simultaneously by rotating and displacing the surface plates and the carrier relative to each other.
Among the above-described two substrates, particularly when the lid substrate is polished, by causing the polishing cloth to mainly slide on a tip end surface of a division wall that surrounds the recessed portion, it is considered possible to improve the surface accuracy of the division wall tip end surface, which is the boding surface with the base substrate.
However, when the lid substrate is polished using the polishing cloth, the polishing cloth wraps around as far as a peripheral edge of the tip end surface (the bonding surface) of the division wall and there is a possibility that a surface sag phenomenon occurs on a peripheral edge portion of the bonded surface. In concrete terms, the bonding surface is formed as a curved surface shape in which a thickness of a glass substrate gets thinner gradually from the center towards the outer peripheral edge side and the inner peripheral edge side. As a result of this, the bonding area with the base substrate becomes smaller, and there is a possibility that the airtightness in the cavity cannot be maintained. More specifically, when it is attempted to bond the two substrates, airtightness in the cavity cannot be secured due to a gap created between the bonding surfaces of the respective substrates, and there is a possibility of deterioration in vibration characteristics of the piezoelectric vibrator.